Electric discharge apparatus



July 15, 1941 J. W. DAWSON ELECTRIC DISCHARGE APPARATUS Filed Aug. 15, 1938 15 Sheets-Sheet 1 4 WITNESSES: \/9/ INVENTOR w A87 John 14/ Dawsazz.

July 15, 1941. J. w. DAWSON 2,248,968

ELECTRIC DISCHARGE APPARQTUS Filed Aug. 15, 1938 3 Sheets-Sheet 2 WITNESSES: INVENTOR J07??? lA/DdZl/SOTZ.

W WM BY y 1941- J. w. DAWSON 2.248968 ELECTRIC DISCHARGE APPARATUS I Filed Aug. '13, 1938 s Sheets-Sheet s INVENTOR J07??? 1M Dawson.

4 BY 3 I 7 ATTORNE WITNESSES:

Patented July 15, 1941 ELECTRIC DISCHARGE APPARATUS John W. Dawson, Auburndale, Mass, assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application August 13, 1938, Serial No. 224,708

9 Claims.

My invention relates to electric discharge apparatus and has particular relation to control apparatus for electric discharge devices.

The invention is the outgrowth of certain improvements made by me in resistance spot and seam welding apparatus. In apparatus of this type intermittent welding impulses are supplied to the material to be welded. To control the magnitude and length of impulses electronic valve means is customarily interposed between the source and the welding load. Since the welding current required is of relatively large magnitudes, the valves used are capable of transmitting substantial current and commonly are of the mercury pool immersed ignition-electrode type.

The welding current is controlled in two respects; as to the time for which each impulse persists and as to the magnitude of the impulse. The former control is known in the art as the timing of the welding current and is ordinarily carried outby so controlling the valve means that it permits current to flow to the welding load during the desired number of periods of the source and blocks the current flow during intervening periods. The other adjustment which is imposed on the welding current is known in the art as heat control and is attained by initiating the flow of welding current at a particular instant in each of the periods during which it flows.

In apparatus constructed and operated in accordance with the teachings of the prior art of which I am aware, two auxiliary valves are used in the control circuit of each main valve for the purpose of attaining the timing and the heat control. Both of the auxiliary valves operate together to release the main valve. The conductivity of one of the auxiliary valves is controlled from a suitable timing device and the conductivity of the other valve from a suitable phase shift element.

Since the welding current required is relatively large and the main valve must, therefore, be capable of carrying considerable current, the associated timing and heat control auxiliary valves are also necessarily relatively large and therefore expensive. The above fact is particularly true where the welding current required is so large that the main valves are of the mercury pool immersed-ignition-electrode type. In this case the ignition current drawn by the main valves is large and although this current flows for only a short interval of time the auxiliary valves must be designed to withstand relatively high surge currents and therefore are expensive.

It is accordingly an object of my invention to provide a welding system having the features of timing and heat control in which the number of auxiliary valves in the control circuits of the main valves shall be a minimum.

Another object of my invention is to eliminate at least one of the auxiliary valves in the control circuit of the main valves or a welding system having the features of timing and heat control.

A further object of my invention is to provide a welding system having the features of timing and heat control incorporating only a single auxiliary valve in the control circuit of each main valve.

A more general object of my invention is to provide apparatus for supplying current impulses of large magnitude for precisely predeterminable intervals of time and of precisely predeterminable magnitude in which the number of valves having a high current rating shall be a minimum.

Another general object of my invention is to provide simple and inexpensive apparatus for supplying current pulses during precisely predeterminable intervals of time and of precisely predeterminable magnitude.

An ancillary object of my invention is to provide apparatus of simple and inexpensive structure for supplying potential impulses for rendering a valve conductive at a precisely predeterminable time.

Another ancillary object of my invention is to provide simple and inexpensive apparatus for obtaining potentials of perpendicular wave front having particular utility in the control of electric discharge valves.

A further ancillary object of my invention is to provide apparatus for supplying power from a source to a load through a pair of electric discharge valves in which physical differences between the valves shall be compensated.

In accordance with my invention, only one auxiliary valve is used in the ignition or control circuit of each main valve and. the timing and the heat control features are provided by controlling these auxiliary valves from at least two additional auxiliary valves connected in series. The latter auxiliary valves may have relatively small rating since they carry only small current. One of the latter valves is now controlled by the timing device and the other from the heat control phase shift network. The timing is attained by applying a continuous gradually decreasing potential to the control circuitof the valve involved, while the heat control is applied in the form of a series of discrete potential impulses, one impulse being applied for each periodic pulsation of the source at the instant in the pulsation at which it is desired that the corresponding main valve be rendered conducting.

The timing potential may be derived in a number of ways but is in accordance with the preferred practice of my invention obtained by charging a capacitor through a resistor of suitable magnitude. The heat control impulses are derived from a periodic source through a phase shift circuit of the usual structure.

The control potentials for the auxiliary valves which still remain in the control circuits of the main valves are in accordance with my invention of perpendicular wave front. The steep wave front potentials are obtained by connecting in series with the input impedance or the primary of the input transformer of the auxiliary valve associated with the main valve, a series transformer. A resistance of suitable magnitude is connected in parallel with the primary of the input transformer of the auxiliary valve. The series transformer is short circuited by the small auxiliary valves and when this occurs a potential is abruptly impressed across the resistor in parallel with the primary of the input transformer and therefore across the primary.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof will best be understood from the following description of speclfic embodiments when read in connection with the accompanying drawings in which:

Figure 1 is a diagrammatic view showing a preferred embodiment of my invention;

Fig. 2 is a graph illustrating the operation of Fig. 1;

Fig. 3 is a diagrammatic view showing a modification of my invention, and

Fig. 4 is a diagrammatic view showing still another modification of my invention.

The apparatus shown in Figure 1 comprises a pair of alternating current line conductors 5 and l which may be ordinary commercial supply conductors providing 60-cycle alternating current. The line conductors supply current to the primary 9 of a welding transformer ll through a pair of main electric discharge devices l3 and I5 of the immersed-ignition-electrode type. Of course, the main discharge devices may also be of any other type such as hot cathode arc-like discharge devices. Each of the main discharge devices l3 and I5 is provided with an anode IT, a mercury pool cathode l8 and an ignition electrode 2|.

Ignition current is provided for the main discharge devices from the main line conductors 5 and 1 through a pair of auxiliary discharge devices 23 and 25 which are specifically shown to be, and preferably are, of the hot cathode arelike type. Each of the auxiliary devices comprises an anode, a hot cathode and a control electrode. The auxiliary devices 23 and 25 are normally non-conductive but are rendered conductive by potential impulses supplied from the main conductors 5 and I through a control transformer 21 having a pair of secondaries 29 and 3|, one of which, 28, is connected between the control electrode 33 and the anode 35 of one of the auxiliary devices 23 through a suitable biasing source 81 and the other one, 3|, is similarly connected to the other device 25.

To start the welding operation, a suitable starting switch 88 such as a push button or a foot pedal is closed. By the operation of the push button a circuit is closed through the exciting coil 4! of a starting relay 43 and the relay is energized. The upper movable contactor 45 of the relay disengages a pair of fixed contacts 41 to open a short circuit 49 across a capacitor 5|. The lowest movable contactor 53 of the relay engages a pair of corresponding fixed contacts 55 and closes a charging, circuit for the capacitor 5| through the anode 51 and the cathode 59 of an auxiliary discharge device if.

The charging discharge device 5| is maintained non-conductive just after its anode cathode circuit is closed by the contactor 53 as its control electrode 68 is connected to its cathode 59 through a resistor 65 and a suitable biasing source 81. However, the central movable contactor 59 of the starting relay 43 engages a pair of fixed contacts II to close a circuit between the control electrode 53 and the cathode 59 which extends through the secondary I3 of a peaking transformer 15. The latter circuit shunts out the resistor 65 between the control electrode 53 and the cathode 58. It extends from the upper terminal ll of the secondary 13 of the transformer 15 through the biasing battery 61, the cathode 59 of the charging device 6|, the control electrode 63 of the device, the central contactor 58 of the starting relay 43 to the lower terminal 18 of the secondary.

The peaking transformer 15 is of a structure particularly adapted for the purpose for which it is used in the present instance. It is described in detail in my copending application, Serial No. 234,635, filed October 12, 1938, and assigned to the Westinghouse Electric & Manufacturing Company. For the present purpose it will be suillcient to say that the transformer comprises a soft-iron U-shaped core 8| which is preferably laminated. Between the cheeks 83 of the core a narrow bar 85 of a highly magnetizable metal, such as Hypernick, extends. The primary 8! of the transformer is preferably adjacent the Hypernick bar 85 and is composed of a relatively high resistant wire. The secondary 13 of the transformer is wound around the primary. The primary terminals are connected to the line conductors 5 and 'l. The transformer is constructed by winding wire on an insulating tube in a mandrel to form the primary, then winding the wire for the secondary over the primary turns and finally removing the wound assembly from the mandrel and inserting the Hypernick bar 85 in the insulating tube. After this the bar and the windings are mounted in the core 8|. Of course the relationship of the windings may be interchanged, the primary may be wound on the outside and the secondary on the inside.

By reason of the presence of the highly magnetizable bar 85 the potentials manifested between the terminals of the secondary of the transformer when a sinusoidal potential is impressed on the primary are of peaked wave form; that is to say, each potential impulse provided at the secondary has a substantial value only during an interval that is short compared to a half cycle of the source. Moreover, by reason of the ohmic resistance in the primary circuit of the transformer, the impulses of substantial magnitude occur early in the half periods of the potential impressed on the primary. In the transformers constructed in accordance with the teachings of the prior art, the impulse of substantial magnitude lags the zero point in the half cycle applied to the primary by 90. In a transformer constructed as described above, the lag is no more than 20 and sometimes considerably less.

Accordingly, when the central contactor 69 of the starting relay 43 is closed, impulses oi peaked wave form are impressed between the control electrode 63 and the cathode 59 of the charging device 6|. The impulses are alternately of negative and positive polarity and of substantial magnitude. When the first impulse of positive polarity is impressed in the control circuit of the charging discharge device the device is rendered conductive and current is transmitted through the charging circuit. The circuit extends from the positive terminal 83 of a voltage divider 9|, supplied with direct current from the line conductors 5 and 1 through a suitable full wave rectifier 93, through a conductor, the lowest movable contactor 53 of the starting relay 43, the anode 51 and the cathode 59 of the charging device 6|, a conductor 91, the capacitor 5|, a resistor 99 in series with the capacitor, to

the negative terminal IOI of the voltage divider. It is to be noted that since the impulses from the transformer can only be supplied early in the half cycles of the potential of the line conductors 5 and 1, the charging of the capacitor 5I begins early in a half period of the source.

As the capacitor 5I charges a gradually decreasing current flows through the series resistor 99. The potential drop across the resistor is applied between the control electrode I03 and the cathode N5 of a timing discharge device I01 which may be of relatively small rating. The control electrode I03 of the timing device I01 is connected to the junction point I 09 of the resistor 99 and the capacitor 5| while its cathode I05 is connected to the adjustable tap III of a voltage divider 9!. Hence, in the absence of charging current through the resistor 99, the control electrode of the timing device is at the potential of the negative terminal IOI of the divider 9|, while the cathode is at the more positive potential of the adjustable tap III. When the capacitor 5| is charging the potential drop across the resistor 99 renders the control electrode I03 more positive than the cathode I05 at least during a portion of the charging time.

The timing device I01 is connected in series with a pair of heat control discharge devices H3 and M5, each of which may also be of relatively small rating. The heat control devices are of the arc-1ike discharge type, each having a control electrode II'I, an anode H9 and a cathode I2I. The anodes N9 of the heat control devices H3 and II5 are connected to the terminals of the secondary I23 of a transformer I25. The primary I21 of the transformer I is connected in series with the primary I29 of the control trans-' former 21 through which the auxiilary devices 23 and 25 in the control circuits of the main devices I3 and I5 are supplied, and a resistor I3I of moderate magnitude is connected in parallel with the latter primary I29. The primaries I21 and I29 are supplied in series from the main line conductors 5 and 1.

The cathodes I2I of the heat control devices H3 and H5 are connected together and the anode I33 of the timing device is connected to the common connection point I35 of the cathodes I2I. The cathode I05 of the timing device I01 is H5 are connected. The potential of the source 5, 1 is thus applied to the series circuit of either of the two heat control devices H3 and Sam the timing device I01. Since initially the heat control devices H3 and H5 are non-conductive, the timing device is at this point also non-conductive, although its control potential is of sumcient magnitude to render it conductive.

Control potential is supplied to the heat control devices H3 and H5 through separate transformers HI and I43. One terminal of a primary I45 of one of the transformers is connected directly to a corresponding terminal of the primary I41 of the other, while the remaining terminals are connected to each other through a suitable resistor I49. Potential is supplied to the transformers HI and I43 from a phase shift network I5I of the usual structure comprising a reactance I53 and an ohmic resistance I55 connected in series with each other and supplied from the main source 5, 1 through still another transformer I 51. As is the usual practice, a voltage divider I59 is connected between intermediate terminals of the reactor I53 and the resistor I55 and the adjustable tap IGI of the voltage divider is connected to the adjustable tap I63 of the resistance I49 connected between the primaries I45 and I41. The intermediate terminal I65 of the secondary I61 of the transformer I51, whereby the phase shift network I5I is supplied is cornected to the junction point I69 of the directly connected terminals of the primaries I45 and I41. By moving the adjustable tap IBI of the voltage divider I59, the phase of the potential impressed on the primaries I45 and I41 relative to the po- F control devices may be rendered conductive if the potential supplied through the transformer I25 to the secondary I23 of which their anodes II9 are connected. Since the timing device I01 is also supplied with suflicient control potential to render it conductive, one or the other of the heat control devices H3 or H5 and the series connected timing device are now simultaneously rendered conductive at instants selected in acof the timing device I01 is at a potential relative to the cathode I05 such that the timing device may be rendered conductive, current is thus supplied through the heat control devices II3 and H5 during alternate half cycles and during this tirne the secondary I23 of the transformer I25 through which the heat control and timing devices are supplied is in part short-clrcuited at predetermined instants in the corresponding half periods. At the instant that the short-circuit occurs potentials are abruptly supplied through the control transformer 21 to the control circuits of the auxiliary devices 23 and 25 in the ignition circuits of the main devices I3 and I5. The auxiliary devices 23 and 25 are, therefore, rendered conductive at instants pre-set by the phase shift network IiI during the half periods during which the timing and heat control devices are conductive, and for intervals beginning at these instants current flows through the main devices I3 and I I and the primary 9 of the welding transformer II and through ,the material I11 to be welded. Any non-uniformity, such as difference in arc drop or promptness of ignition which exists in the main valves I3 and I5 or their ancillary circuits is compensated by the proper adjustment of the vari-.

able tap I63 of the resistor I39.

Since the timing device I91 is connected in series with the heat control devices III and II! to the source 5, 1, it is essential that both the timing device and either one of the heat control devices be capable of conducting current, and therefore, the discharge devices pass current only so long as the charging current for the capacitor 5I is suificient to neutralize the bias potential provided by the voltage divider 9| and to render the timing device I01 conductive. The time required for charging the capacitor II thus determines the time during which the heat control devices H3 and II! and the series connected timing device are capable of conducting at all and, therefore, the time during which welding current may flow. On the other hand, the heat control devices prevent conduction until the instant pre-set by the phase shiftnetwork ISI, and thus they determine the amount of current which may flow and therefore the amount of heat supplied by the current for welding. It is to be noted that in accordance with my invention, only one timing device of small rating and two heat control devices, also of small rating, are necessary. The three small devices replace at least two heat control devices of large rating required in accordance with the teaching of the prior art.

The application of starting pulses to the charging device 6| early in the half periods of the supply source 5, 1 is essential for the proper operation of my invention because it is important to make certain that the welding current should flow during every half period of the selected number of half periods. This would not be the case if the starting pulse for the charging device BI were applied later in the selected half period of the source. In such a case it would often happen that the starting pulse for the charging device is applied at a later phase point than that at which the impulse supplied during the same half period by the phase shift network I5I to the heat control devices rises to a sufficient value to render one of the latter devices conductive. In such an event, current flow through the welding load would be initiated during the first half period later than during the other half periods and an unbalance would be produced.

The relationship between the potentials supplied to control the main discharge devices I3 and I5 is graphically illustrated in Fig. 2. The

that it may happen at heavy sine wave I19 represents the potential supplied by the main source 5, 1 to the series network of the primaries I21 and I29 of the two transformers I23 and 21, respectively. The sine curves I9I and I83, partially in light full lines I" and I31 and partially in light broken lines I99 and I9I, represent the potentials supplied to the individual primaries I21 and I29, respectively, as a function of time when the heat control discharge devices I I3 and II I are non-conductive. The lower curve I93 represents the potential impressed across the resistor I3I and therefore across the primary I29. The upper curve III represents the potential impressed across the primary I21. The vector sum of the ordinates of the two curves is at every point equal to the ordinate of the heavy curve. It is to be noted that since secondaries 29 and 3I of transformer 21 are open just as the secondary I23 of transformer I25, the resistor I31 shunting the primary I29 of the former must be of such magnitude that its impedance is small compared to the impedance of transformer I 2!. Otherwise a substantial portion of the potential of source I, 'I will be absorbed across the resistor and a relatively large potential will be impressed in the control circuits of auxiliary valves 23 and 2|. Under such circumstances, the latter valves would tend to become prematurely conductive.

What occurs when the heat control devices III and II 5 and the timing device I31 are conductive is graphically illustrated by the curve I93 drawn in very heavy lines and the curve I" in very light lines. when the secondary I23 of the transformer I 25 supplying the heat control devices I I3 and II I is short circuited by the devices, a greater portion of the total potential from the source 5, 1 is abruptly transferred to the terminals of the resistor I 3| and therefore to the terminals of the primary I29 of the other transformer 21. This is represented by the upper curve I93. The heavy vertical portion line I91 of the curve I93 arising from the lower light curve I83 represents the abrupt rise in the potential impressed across the control transformer 21 which occurs. As seen the potential rises from the small value of curve I09 to the large value at the upper terminal of the vertical line I91. The same vertical line I91 from the upper light curve I 9| to the lower very light curve I93 represents the drop which occurs in the potential impressed across the primary of the other transformer I" when the heat control devices H3 and III and the timing device I01 are conductive. What happens during one half cycle of the source I, 1 of course also happens during the subsequent one which is of opposite polarity as shown in Fig. 2. The secondaries 29 and 3I are so connected that both half cycles are properly utilized. It is seen that the curves I33 and I93 representing the potential impressed on the primary I29 of the control transformer 21 and, therefore, the potentials I in the control circuits of the auxiliary devices 23 and 25 of perpendicular wave front and therefore, the control potentials for the'auxiliary devices have a similar wave front and the auxiliary devices are energized precisely at the desired instants.

An analysis of the Fig. 1 apparatus will reveal times that the welding current may not flow during the last half cycle of a number of half cycles for which the apparatus is set. This may occur, for example, if the charging current through the resistor 99 in series with the capacitor SI falls to too low a value to maintain the conductivity of the timing device I01 early in the last desired conductive half period of the source 5, The timing device remains non-conductive until the heat control devices H3 and H5 are rendered conductive and potential supplied work I5I to the heat control devices may rise to a value above the critical potential of the heat control devices later in the last half period than the instant at which the potential supplied by the resistor falls below the necessary value. It may happen, therefore, that the possibility'or rendering the timing device I01 conductive shall have ceased when the possibility of rendering a heat control device H3 or H5 conductive arises. In such an event the necessary short circuiting of the transformer I25 does not occur during the last half period and, therefore, the main devices load during very often and therefore I is satisfactory for the common run of welding loads. However, sometimes greater perfection in the welds is required and where this is the case the apparatus shown in Fig. 3 is found to be satisfactory.

In the Fig. 3 apparatus, the timing discharge device I01 is connected to the terminals of the secondary I23 whereby the heat control and timing devices H3, H5 and I01 are supplied through the branches I99--2I)0, and 2IlI-20II of a full wave rectifier 203, or through separate rectifiers, and through a resistor 2B5. 21 of the full wave rectifier I are connected to control devices H3 the cathode 203 is connected to the i33 of the timing device I01 through the resistor 205. When energizing potential is now supplied to the timing device during any point in any half period of the source 5, 1, the timing device is rendered conductive and current flows from the secondary of the transformer through the rectifier 203 and the resistor 205. The resistor 205 is of such magnitude that the current flow is so small that for all practical purposes the transformer I25 supplying t .e current flow may be regarded as still open circuited. Since the timing device I01 is of the arc-like discharge type, it remains conductive during the remainder of the half period during which it is rendered conductive when once rendered conductive. Accordingly, if it should happen that the potential supplied through the resistor 99 in series with the capacitor 5I is sufficient to render the timing device conductive early in the last of a series of half periods and then immediately falls below that value sufilcient for conductivity, the timing device I01 will remain conductive and, therefore, when subsequently the energizing potential is supplied to one or the other of the heat control devices H3 or II5 conduction through the heat control devices and the timing device takes place and the transformer is short circuited in the usual manner.

The apparatus shown in Fig. 4 is a modification of the Fig. 1 system In this case resistors connected between the cathodes m of the heat control discharge devices H3 and H5 and the anode I33 of the timing device III1 is connected to the intermediate terminal 2H with which the resistor is provided. The resistors are connected directly across the primary I28 0! the control transformer 21 in lieu of the resistor I in the other modifications. However, the primary I29 is not connected to the source through the primary I21 oi transformer I25 in this case;

from the phase shift netthe primary I21 alone is connected across the source.

In this arrangement, the potential drop across the resistors 201 or 209 is the potential impressed on the transformer 21 and therefore in the control circuits of the auxiliary devices 23 and 25. Accordingly, when the heat control devices H3 and H5 and the timing device I01 are nonconductive, the potential impressed on the control transformer 21 is zero. When the heat control and timing devices are conductive, the potential drop produced by the current flow is imthe control transformer 21 and the and 25 are rendered conductive. As the conductivity of the devices is initiated at selected points in the hall periods of the sources 5 and 1, the devices 23 and 25 and the main devices I3 and I5 are rendered conductive at the selected points in the half periods. Of course, as in the other modifications, the main valves I3 and I5 may be rendered conductive at different points in the hall! periods to compensate for diiierences in their structure and operation.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereoi are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. For use in controlling the current flow from a source of periodically pulsating current the combination comprising electric valve means connected to said source and said load, means for continuously impressing a potential for a predetermined interval of time for rendering said valve means conductive during a predetermined number oi pulsations of said source corresponding to said interval, said potential impressing means comprising a capacitor, and means for charging said capacitor, and means for impressing impulses on said valve means to render said valve means conductive at predetermined points in each of the said predetermined number of polsations, said valve means comprising at least two groups of electric discharge devices and said source having a pair intermediate tap. one of said groups including a pair oi asymmetrically conductive electric disdischarge type. each device having a plurality of principal electrodes, a principal electrode of each of said of discharge devices being connected to a terminal tap and another principal electrode of one or said pair of devices being connected to a corof the other, the other group including a single discharge device having principal electrodes connected between said intermediate tap and the connection point electrodes of said pair of discharge devices and said impulses being impressed on said first mentioned group of discharge devices and said potential being impressed on the discharge device of said lastnamed group, and asymmetrically conductive means having a high impedance compared tr that of said asymmetrically conductive device: lay-passing each of said asymmetrically conduc tive devices.

2. For using in supplying a load from a sourc of periodically pulsating current the combinatio electric valve means interposed be tween said source and said load, said valve meai comprising a plurality of main electric dischar devices, each having a control circuit, including devices, each having a control circuit, including an auxiliary discharge device, means for iman auxiliary discharge device, means for impresspressing potentials for a predetermined interval ing potentials for a predetermined interval oi of time on said auxiliary discharge devices for time on said auxiliary discharge devices for renrendering said valve means conductive during a d dering said valve means conductive during a pretial impressing means comprising at least two pressing means comprising at least two additional additional electric discharge devices each having electric discharge devices each having a control a control electrode and a plurality oi principal 1o electrode and a plurality of principal electrodes,

tinuously impressing a potential during said inuously impressing a potential during said interval between the control electrode and a printerval between the control electrode and a principal electrode of one of said discharge devices, cipal electrode oi one 0i said discharge devices, and means for impressing impulses between a and means for impressing impulses between a control electrode and a principal electrode 01' control electrode and a principal electrode 01' the the other of said last-named discharge devices other of said last-named discharge devices to to render said other discharge device conductive render said other discharge device conductive at at predetermined instants in said predetermined predetermined instants in said predetermined periodically pulsating current the combination ing the last of said pulsations regardless of how ry tric discharge means to be rendered conductive 6. For use in supplying a load from a source or potential and an impulse potential and when so comprising electric valve means interposed berendered conductive to control said valve means, tween said source and said load, said valve n are discharging said capacitor, for impressing said concharge devices each having an ignition circuit, tinuous potential on said auxiliary discharge means for impressing potentials for a predetermeans during predetermined pulsations of said 5 mined interval of time in said auxiliary ignition source, said mpressin ans inclu circuits for rendering said valve means conducmeans insuring that the impressing of said contive during a predetermined n mber of p muous potenti begins ear the begin oi ons of said source corresponding t ai interthe first of said predetermined pulsations or said val, said po ential impressing means compri ng u and me ns :0 impressing aid impulse 40 at least two groups of auxiliary electric discharge potential on said auxiliary discharge means durdevices, an auxiliary source having a pai of tering the time that said potential is impressed minal taps and an intermediate tap, one of said thereon to render said valve means conductive groups including a p r 1' le tric discharge deat predetermined instants in said pulsations. V1688 each device IIii-V1118 8 plurality 01' Principal the other group including a single discharge desource, said impressing means also including capacitor and means for charging said ca aci r means insuring that the impressing of said conr mpr ssi p ential continuously during said nuous potent al n th begjnnj g of interval on the dischar device oi said lasthe rst of said predetermined pulsations of said named group, said devices of said groups being source, and means for impressing said impulse rendered conductive when said se potential potential on said auxiliary discharge means dur- 1 i c inuous potential are im ressed mg the me a said potential is impressed r u e in s pplying a load from a source 01' thereon to render said valve means conductive at e i y n current the co nation at which the impressing of said potential begins. ing potentials for a predetermined interval of 5 For use in supplying a load from a source of time on said auxiliary discharge devices for renperiodically pulsating current the combination dering said valve means conductive during a pre comprising electric valve means interposed bedetermined num r of pul tions or said source sa tween said source and said load, said valve means corresponding to said in erval, said potential im comprising a plurality of main electric discharge pressing means comprising at least two addl.

tional electric discharge devices each having a control electrode and a plurality of principal electrodes, the principal electrodes of said last named discharge devices being connected in series with each other, means for continuously impressing a potential during said interval between the control electrode and a principal electrode of one of said discharge devices, and means for impressing impulses between a control electrode and a principal electrode of theother of said lastnamed discharge devices to render said other discharge device conductive at predetermined instants in said predetermined, number of pulsations, and high impedance means for by-passing said other discharge device.

8. For use in supplying a load from a source of periodically pulsating current the combination comprising electric valve means interposed between said source and said load, said valve means comprising a plurality of main electric discharge devices, each having a control circuit including an auxiliary discharge device, means for impressing potentials for a predetermined interval of time on said auxiliary discharge devices for rendering said valve means conductive during a predetermined number of pulsations of said source corresponding to said interval, said potential impressing means comprising at least two additional electric discharge devices each having a control electrode and a plurality of principal electrodes, the principal electrodes of said last named discharge devices being connected in Se ries with each other and one of said devices being asymmetrically conductive, means for continuously impressing a potential during said interval between the control electrode and a principal electrode of said other of said discharge devices,

ahd means for impressing impulses between a control electrode and a principal electrode of one of said discharge devices to render said discharge device conductive at predetermined instants in said predetermined number of pulsations, and asymmetrically conductive means having a high impedance compared to the impedance of said asymmetrically conductive discharge device for by-passing said asymmetrically conductive devices.

9. For use in supplying a load from a source of periodically pulsating current the combination comprising electric valve means interposed between said source and said load, said valve means comprising a plurality of main electric discharge devices, each having a control circuit including auxiliary discharge devices, means including a capacitor and means for charging said capacitor, for impressing a potential on one of said auxiliary discharge devices for rendering said auxiliary discharge devices conductive during predetermined pulsations of said source and means for impressing impulses on others of said auxiliary discharge devices during the time that said potential is impressed on said one device to render said auxiliary discharge devices conductive at predetermined instants in said pulsations, thereby to render said valve means conductive at said predetermined instants, and means including a rectifier and an impedance for assuring that said auxiliary discharge devices are rendered conductive during the last of said pulsations regardless oi! how late in said pulsation the corresponding impulse is impressed thereby assuring that said valve means is rendered conductive during said last pulsation.

JOHN W. DAWSON. 

